Recently, with the development of portable devices, such as portable phones, laptop computers, and camcorders, the demand for small-sized secondary batteries such as lithium secondary batteries has increased. In particular, a vast amount of research has been conducted into lithium secondary batteries using lithium and a non-aqueous solvent serving as an electrolyte because it is highly likely to form small-sized, lightweight batteries having a high energy density.
Positive electrode active materials of lithium secondary batteries include lithium cobalt oxide (LCO)-based materials (e.g., LiCoO2), nickel cobalt manganese (NCM)-based materials (e.g., LiNi1/3Co1/3Mn1/3O2 or LiNi0.5Co0.2Mn0.3O2), and OLO-based materials, which have layered crystal structures, LMO-based materials (e.g., LiMn2O4) having spinel crystal structures, and lithium iron phosphate (LFP)-based materials (e.g., LiMPO4) having olivine structures.
The LCO-based materials, which are typical positive electrode active materials, have excellent lifespans and conductive characteristics but have small capacities and require expensive raw materials. Also, since the NCM-based positive electrode active materials can obtain neither high charge/discharge efficiency nor high temperature characteristics, the NCM-based materials do not have reliable battery safety yet.
In addition, the LFP-based materials are very stable at high temperatures and have attracted much attention as inexpensive positive electrode active materials because the LFP-based materials do not require high-priced elements, such as cobalt or nickel. However, when a battery is configured by bonding a positive electrode formed using an LFP-based material as an active material with an electrolyte, a negative electrode, and a separator, a transition metal M may be eluted into the electrolyte. Also, when the eluted transition metal reacts with the electrolyte and generates gases, battery safety may be threatened. Furthermore, when the eluted transition metal is precipitated in a metal phase on the opposite negative electrode, intercalation and deintercalation of lithium ions may be hindered, thereby causing a voltage drop or shortening the lifespan of the battery.
Thus, many attempts have been made to improve lifespan characteristics of positive electrode active materials for lithium secondary batteries in the field of batteries, and there is still a demand for positive electrode active materials with improved lifespan characteristics.
Among such attempts, to increase the lifespan characteristics of the positive electrode active material for the lithium secondary battery, a method of forming a metal oxide coating layer 30 on the surface of a positive electrode active material 10 by blending the positive electrode active material 10 with a metal oxide precursor 20 has been proposed as shown in FIG. 1. However, this method may actually lower efficiency because the metal oxide coating layer 30 formed on the surface of the positive electrode active material 10 does not have ionic conductivity.
Therefore, the present inventors discovered that when a porous metal oxide containing a material having lithium ion conductivity is coated on a conventional positive electrode active material, conduction of lithium ions between an external electrolyte and the positive electrode active material under coating layer may be facilitated, thereby maintaining lifespan characteristics and improving efficiency, and thus completed the present invention.